The invention relates to a vehicle fuel cell system having a burner for producing a heat flow by combustion of a fuel gas which reacts with an oxidant.
In vehicles which are equipped with conventional internal combustion engines, the passenger compartment of the vehicle is generally heated by waste heat from a coolant circuit that cools the internal combustion engine. In particular, the passenger compartment of the vehicle is actually heated by a heating heat exchanger which is arranged in the coolant circuit. This arrangement has the disadvantage, however, that the coolant circuit heats up only comparatively slowly after the internal combustion engine is started, and rapid heating of the passenger compartment of the vehicle, as is desirable in particular in winter temperature conditions, is not possible.
Further problems result with drive concepts based on the use of fuel cells. For example, when the outside temperatures are below 0° C., it is possible for the process water in the fuel cell to freeze, impeding the gas-diffusion processes required within the fuel cell in order to generate electricity. In this case, cold starting of the fuel cell system is feasible only in a restricted form.
In this context, German patent document 103 24 213 A1 discloses a fuel cell system in which a hydrogen burner produces a heat flow that is used to preheat a fuel cell, by an exothermic reaction of hydrogen with air oxygen. According to one described embodiment of the fuel cell system, the heat flow that is produced is also used to heat the passenger compartment of the vehicle. No provision is made to use the process heat that is created during operation of the fuel cell system.
One object of the present invention therefore is to improve the efficiency both for preheating of the fuel cell system and for heating of the passenger compartment of the vehicle.
This and other objects and advantages are achieved by the fuel cell system according to the invention, which includes, in addition to a conventional fuel cell, a separate burner that produces a heat flow by combustion of a fuel gas which reacts with an oxidant. The heat flow is used to preheat the fuel cell and to heat the passenger compartment of the vehicle. According to the invention, a heating heat exchanger in the form of a radiator or the like is provided in order to heat the passenger compartment of the vehicle; the heat exchanger is arranged in a coolant circuit for cooling the fuel cell, and is externally heated, at least at times, by means of the burner.
After starting up the fuel cell system, the external heating can be carried out at least until the coolant which is circulating in the coolant circuit has reached its normal operating temperature. The heating heat exchanger is then heated in the normal manner by use of the process heat created during operation of the fuel cell, which is dissipated via the coolant circuit.
The external heating of the heating heat exchanger makes it possible to operate the heating heat exchanger (and therefore, to heat the passenger compartment of the vehicle) independently of the instantaneous operating state of the fuel cell system. On the other hand, the fuel cell can be preheated, in the opposite direction, by the thermal coupling between the heating heat exchanger and the coolant circuit. This is particularly important, for cold starting of the fuel cell system when the outside temperatures are close to freezing point.
There are various options for external heating of the heating heat exchanger. For example, it is possible to arrange a heating heat exchanger in the coolant circuit of the fuel cell system downstream from the burner. The circulation of coolant in the coolant circuit transports at least some of the combustion heat produced by the burner from the heat exchanger to the heating heat exchanger.
Alternatively, the burner can be followed by a heat exchanger which is connected to the heating heat exchanger either directly or else via its own coolant circuit. In this case, the heating heat exchanger can be heated independently of the operation of the fuel cell or of the coolant circuit associated with the fuel cell.
In order to provide such “stationary heating”, the burner has its own associated oxidant and/or fuel gas supply.
In particular, the oxidant supply is a small electrical fan which is operated by means of a starter battery located in the vehicle. A reservoir is also provided for the fuel gas supply and is filled with fuel gas from a high-pressure tank or a reformer during operation of the fuel cell system, in order to allow continuous further supply of fuel gas to the burner once the fuel cell system has been switched off.
The fuel cells normally used in the motor vehicle field produce electrical power by electrochemical reaction of an oxygen-containing oxidant with a fuel gas containing hydrogen. This includes in particular fuel cells with a polymer membrane (polymer exchange membrane fuel cell or polymer electrolyte fuel cell) which are operated at comparatively low operating temperatures in the range from 70 to 200° C. In order to minimize the complexity, the burner also uses the same oxidant containing oxygen and/or the same gas containing hydrogen.
The oxidant is typically air taken from the atmosphere outside the vehicle, via an air filter system which has a silencer in addition to a mechanical and/or chemical particle filter. However, it is also feasible to supply the burner with the cathode off-gas (which contains oxygen and is created during operation of the fuel cell system) as the oxidant. In this case, there is no need for a separate oxidant supply, this can be provided exclusively for stationary heating purposes.
The burner itself may be either a catalytic burner or a flame or pore burner, which offers the advantage of particularly high heating power, while a catalytic burner is characterized by controlled combustion without any residue.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.